This invention relates to a high color rendering fluorescent lamp having radiation color temperature of 4200.about.5600 K. (Kelvin) and having average color rendering index (Ra) of not less than 95.
Calcium halophosphate phosphors activated with antimony and manganese are widely used for illumination fluorescent lamps for the general use. However, due to its poor color rendering, the use thereof is inadequate at places such as art museums and color printing factories where a high color rendering is required.
There are known heretofore the following measures in order to solve the problem and achieve a high color rendering: In one example, the emitting layer comprises a mixture of several kinds of phosphors so as to approximate the spectral energy distribution of the reference illuminant. In another example, mercury line at the visible short wavelength region hindering good color rendering is suppressed.
Of the above, for the so-called EDL Type lamps that is required to have especially a high color rendering, suppression of mercury line is effective for the improvement in color rendering. For this purpose, there are known a lamp having a double coated phosphor layer or one in which a separate pigment layer is provided.
The former concerns a phosphor layer in which emission energy at 405 and 436 nm mercury line in the visible short wavelength region that hinders improvement in the color rendering is absorbed and, moreover, the absorbed energy itself is converted to the red color radiation; for example, a phosphor combination comprising a first layer of magnesium fluorogermanate activated with manganese (3.5MgO, 0.5MgF.sub.2, GeO.sub.2 :Mn) and a second layer of another phosphor coated thereon (Japanese Patent Publication No. 9868/1966).
The latter is to provide a yellow pigment layer between a glass tube and a phosphor, whereby about 60% of 436 nm mercury line may be suppressed as compared with one in which no pigment layer is provided (Japanese Patent Publication No. 15896/1973). However, the known methods described above involve such disadvantages as production inefficiency due to complication of lamp manufacturing steps, variation of luminous color and color rendering caused by change in the thickness of pigment layer, or lowering of phosphor efficiency caused by absorption of emission energy.
Recently, improvements in the above-mentioned problems have been proposed (Japanese Laid-Open Patent Application Nos. 102073/1979 and 115489/1980). They fail, however, to fully improve the problems.
According to (Japanese Laid-Open Patent Application No. 102073/1979, at least four different species of phosphors including calcium halophosphate and zinc silicate phosphor must be used because the peak wavelength of the blue-green phosphor employed is too short to bring chromaticity coordinate upon blackbody locus. This inevitably gives rise to such disadvantages as considerable variation of luminescent color and color rendering, and lowering of efficiency.
The latter of the above-mentioned Japanese patent documents also relates to such advantages as high efficiency and superior color rendering. However, the disclosed improvement brings about difficulty in color control such as luminescent color and color rendering because three different species of phosphors, including blue and red color phosphors emitting within a narrow wavelength region, must be used admixed.